The invention relates generally to x-ray tubes, and more particularly to structures for emitters utilized in an x-ray tube that exerts thermal expansion and high centrifugal force stresses on the emitter.
X-ray systems may include an x-ray tube, a detector, and a support structure for the x-ray tube and the detector. In operation, an imaging table, on which an object is positioned, may be located between the x-ray tube and the detector. The x-ray tube typically emits radiation, such as x-rays, toward the object. The radiation, passes through the object on the imaging table and impinges on the detector. As radiation passes through the object, internal structures of the object cause spatial variances in the radiation received at the detector. The detector then emits data received, and the system translates the radiation variances into an image, which may be used to evaluate the internal structure of the object. The object may include, but is not limited to, a patient in a medical imaging procedure and an inanimate object as in, for instance, a package in an x-ray scanner or computed tomography (CT) package scanner.
Presently available medical X-ray tubes typically include a cathode assembly having an emitter thereon. The cathode assembly is oriented to face an X-ray tube anode, or target, which is typically a planar metal or composite structure. The space within the X-ray tube between the cathode and anode is evacuated.
The emitter functions as an electron source that releases electrons at high acceleration. Some of the released electrons may impact the target anode. The collision of the electrons with the target anode produces X-rays, which may be used in a variety of medical devices such as computed tomography (CT) imaging systems, X-ray scanners, and so forth. In thermionic cathode systems, an emitter is included that may be induced to release electrons through the thermionic effect, i.e. in response to being heated. This emitter is often a flat surface emitter (or a ‘flat emitter’) that is positioned on the cathode with the flat surface positioned orthogonal to the anode, such as that disclosed in U.S. Pat. No. 8,831,178, incorporated herein by reference in its entirety for all purposes. In the '178 patent a flat emitter with a rectangular emission area is formed with a very thin material having electrodes attached thereto, which can be significantly less costly to manufacture compared to emitters formed of wound (cylindrical or non-cylindrical) filaments and may have a relaxed placement tolerance when compared to a wound filament emitter.
Typical flat emitters are formed with an electron emissive material, such as tungsten, having a flat electron emission surface divided by slots with a number of interconnects to create either a single meandering current carrying path including a number of spaced but interconnected ribbons, or multiple parallel current carrying paths, that generate electrons when heated above some temperature. Current is directly applied from the cathode through the flat emitter to generate heat in the emitter and results in the emitter surface reaching temperatures high enough to produce electron emission, typically above 2000° C.
Typical flat emitters are not capable of operating in the regime of combined long emissive lengths, high emission temperatures, and high acceleration forces. In particular, long emissive lengths for the flat emission surface and high accelerations increase the stress beyond the strength available in the emitter material at high emission temperatures. When the X-ray tube is rotated around the object being imaged, the centrifugal forces exerted on the emitter can be in excess of 30 G. Further, flat emitters operate at temperatures above 2000° C. to produce the necessary electron emission for a satisfactory resolution of the X-ray image of the object. At these extreme temperatures the properties of the material forming the emitter, such as creep resistance and yield strength, are greatly reduced from room temperature values. The high operating temperatures at which the emitter is operated also induce thermal strains due to the thermal expansion of the emitter exceeding the thermal expansion of the lower temperature sub-structure. For long flat emitters operating at high temperatures with high centrifugal acceleration force exerted on the emitter, the combination of the high centrifugal force, thermal strains, and reduced material properties results in the emitter deforming in the direction of the centrifugal force, which can cause the slots dividing the emission surface to close, such that adjacent ribbons come into contact with one another. A closed slot creates an electrical short, reducing the temperature of the emission area and impacting the emission profile of the emitter.
As a result, it is desirable to develop a structure and method for use of a flat emitter of an x-ray tube that is designed to accommodate for the high centrifugal force, thermal strains, and reduced material properties of the material forming the emitter thus minimizing any structural alteration or deformation of the emitter when in use over the life of the emitter.